Two necessary conditions for validation of the hypothesis that changes in the chain length of the naturally occurring poly-Gamma-glutamyl derivatives of the folate co-enzymes play a role in the regulation of one-carbon metabolism are now well established: a) the catalytic efficiency of most folate-dependent enzymes is affected in vitro by the chain length of the folate coenzymes; and b) changes in the chain length distribution of the folates occur in vivo in response to alterations in the steady-state of one-carbon metabolism. It remains to establish how, and in response to which effectors these changes in the folate co-factors occur. Of the two opposing enzyme systems that modify the poly-Gamma-glutamyl chain, the folylpolyglutamate synthetase (FPGS) and the pteroylpolyglutamate hydrolases (the conjugases), the metabolic role or the latter is the least understood. We hypothezise that endo-conjugases, primarily hepatic, are involved in the mobilization of folate stores by degrading in one step the intracellular 5-CH3-H4PteGlun to the circulating 5-CH3-H4Pte-monoglutamate. Exo-conjugases, removing one Glu residue at a time, are believed (together with FPGS) to be involved in the regulation of one-carbon metabolism via midification of the poly-Gamma-glutamyl chain length. To test this hypothesis we propose the following Specific Aims: 1) to continue the development of methods for the study of endo- and exo-conjugases, including specific assays for each of these activities; 2) to study changes in the activity of endo- and exo-conjugases during situations that alter the steady-state of 1-C metabolism; 3) to study the effects of increased folate demand by extrahepatic tissues on endo- and exo-conjugase activities of the liver; and 4) to attempt the purification of endo- and exo-conjugases from rat liver and kidney and study the properties of the purified enzymes with special attention to the study of effectors.